Feeding three different diets resulted in significant changes in the bacterial community found in the dung of horses, researchers have found.

The gastrointestinal tract of mammals contained an extensive microbial population, providing the host with essential nutrients, as well as modulating the immune system.

In the horse, short chain fatty acids produced by microbial fermentation of fibre in the large intestine provide a significant proportion of the animal’s daily energy requirements.

The British and American researchers who conducted the study said little was still known about the overall composition of the microbial community in the equine hindgut, despite the importance of microbes in supplying energy.

Several recent studies have characterised the faecal bacterial community of horses, but none had investigated the effect of diet on the gut microbiota, they said.

The researchers, whose findings have been published in the open-access journal, PLoS ONE, collected dung samples from 17 horses, each fed three different diets: high fibre (forage only); high fibre with a starch-rich supplement; and high fibre with an oil-rich supplement.

They used DNA analysis techniques to investigate the microbial community, or microbiome, in the manure of the horses, all of which were kept at Michigan State University.

The scientists also looked into the effect of age on the make-up of the microbiome, comparing the eight mature horses, aged 5–12, in the study with the nine elderly horses, aged 19–28.

They found a reduction in bacterial diversity in the elderly horse group and significant differences in the bacterial communities between each diet.

Most of the differences found were related to the Firmucutes phylum, with some changes in Bacteroidetes, Proteobacteria, Actinobacteria and Spirochaetes.

They found that the core bacteria – that is, those present in all horses – associated with the oil-rich supplemented diet was somewhat smaller than the high-fibre forage-only diet, but the core associated with the starch-rich supplemented diet was even smaller.

They found that the core associated with samples across all three diets was extremely small, dominated by the order Clostridiales, with the most abundant family being Lachnospiraceae.

“The presence of such a small core may begin to explain why the horse is so susceptible to metabolic dysfunction,” they said.

Further, there was a lack of obvious “key” members in the core.

“The reduction in core size when horses are fed a diet other than one high in fibre and, particularly when fed a high starch supplemented diet, may increase the risk for subsequent metabolic dysfunction,” they said.

However, while the core was extremely small, the overall bacterial community within the faeces of horses on the different diets was found to be highly diverse, but significantly less so in the older horses.

All horses remained healthy throughout the study with no gastrointestinal disturbances.

The researchers concluded that forage-based diets with starch-rich or oil-rich complementary feeds were associated with differences in the faecal bacterial community compared with forage alone.

Further, as observed in people, ageing was associated with a reduction in bacterial diversity. However, there was no change in the bacterial community structure in these healthy animals associated with age.